Method of treating the surface of iron alloy materials

ABSTRACT

A layer of a nitride or carbonitride containing at least one of chromium, Group Va elements, titanium and zirconium is formed on the surface of an iron alloy material which has been nitrided. The layer is formed by heating the iron alloy material at a temperature not exceeding 700° C. with a material containing at least one of chromium, Group Va elements, titanium and zirconium and a treating agent. It is a dense layer bonded tightly to the iron alloy material. As a low temperature not exceeding 700° C. is employed, no large amount of heat energy is required, nor is any thermal strain produced in the iron alloy material.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a method of treating the surface of an ironalloy material, as for making dies, jigs, tools or machine parts, toform thereon a layer composed of a nitride or carbonitride of one ormore of the surface layer-forming elements, i.e., chromium, Group Vametals (vanadium, niobium and tantalum), titanium and zirconium.

2. Description of the Prior Art

It is well known that if the surface of an iron alloy material iscovered with a layer composed of a carbide, nitride or carbonitride ofone or more of the surface layer-forming elements, it improves the wear,seizure, oxidation and corrosion resistances and other properties of theiron alloy material. There have been proposed many methods for formingsuch a surface layer.

For example, a layer of chromium carbide is formed on a iron alloymaterial by immersion in a salt bath composed of a chloride (seeJapanese Laid-Open Patent Specification No. 200555/1982 or 197264/1983).An iron alloy material which has been nitrided is chromized, whereby alayer of chromium carbonitride is formed on its surface (see JapanesePatent Publication No. 24967/1967 or U.S. Pat. No. 4,242,151).

Both of these methods, however, involve heating at a temperature whichis higher than the Ac₁ transformation point of iron (about 700° C.). Theheat develops strain in the iron alloy material and if it has acomplicated shape, it is very likely to crack. The use of such a hightemperature presents other problems, too, including the worsening of theworking environment.

There have, therefore, been proposed a number of methods for forming asurface layer at a temperature not exceeding 700° C. They employ, forexample, a halide of the surface layer-forming element or elements andform a surface layer by, for example, CVD (chemical vapor deposition),plasma CVD, ion plating or PVD (physical vapor deposition) [see, forexample, Japanese Laid-Open Patent Specification No. 65357/1980,154563/1980 or 151469/1983, "Kinzoku Hyomen Gijutsu" (Metal SurfaceTechnology), No. 2, p.28 (1979), or Japanese Laid-Open PatentSpecification No. 2715/1980 or 164072/1980].

Although all of these methods can form a surface layer on an iron alloymaterial without producing any thermal strain, they can hardly form asurface layer which is satisfactory from the standpoints of thicknessuniformity and adhesive strength. All of them are complicated andrequire expensive equipment. Moreover, they are inefficient, as theyneed be carried out in the presence of hydrogen or at a reducedpressure.

SUMMARY OF THE INVENTION

It is, therefore, an object of this invention to provide a method whichcan form on the surface of an iron alloy material a layer composed of anitride or carbonitride of one or more surface layer-forming elementsand adhering strongly to the base material efficiently at a lowtemperature by means of a very simple apparatus without producing anythermal strain in the base material.

It is another object of this invention to provide a method which canform such a surface layer at a low temperature without requiring a largeamount of energy, and which is, therefore, easy to carry out.

These objects are attained by a method which comprises nitriding an ironalloy material to form on its surface a layer of a nitride composed ofiron and nitrogen, or iron, carbon and nitrogen, and heating the ironalloy material at a temperature not exceeding 700° C. with a materialcontaining one or more surface layer-forming elements selected from thegroup consisting of chromium, Group Va metals, titanium and zirconium,and a treating agent to diffuse the surface layer-forming element orelements into the surface of the iron alloy material to form thereon alayer composed of a nitride or carbonitride of the surface layer-formingelement or elements, the treating agent being composed of at least oneof the chlorides, borofluorides, fluorides, oxides, bromides, iodides,carbonates, nitrates and borates of alkali and alkaline earth metalsand/or an ammonium halide or a metal halide or both.

Other objects, features and advantages of this invention will becomeapparent from the following detailed description and the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1, 13, 25 and 32 are graphs showing the thickness of the surfacelayers formed in EXAMPLES 1, 13, 26 and 38, respectively, in relation tothe immersing time employed;

FIGS. 2, 4, 6, 14, 16, 19, 26, 28, 30, 33, 35, 37 and 38 arephotomicrographs of 400 magnifications showing the structure in profileof the surface layers formed in EXAMPLES 1 to 3, 13, 14, 18, 26, 27, 31,38, 39, 43 and 44, respectively; and

FIGS. 3, 5, 7 to 12, 15, 17, 18, 20 to 24, 27, 29, 31, 34, 36 and 39 aregraphs showing the results of analysis by an X-ray microanalyzer of thesurface portions of the iron alloy materials as treated in EXAMPLES 1 to4, 7, 8, 10, 11, 13, 14, 16, 19 to 21, 23, 24, 26, 27, 35, 38, 39 and47, respectively.

DETAILED DESCRIPTION OF THE INVENTION

According to this invention, a layer composed of a nitride orcarbonitride of one or more surface layer-forming elements is formed onthe surface of an iron alloy material. The term "iron alloy material" asherein used means not only a material containing carbon, such as carbonor alloy steel, cast iron or a sintered alloy, but also a materialhardly containing carbon, such as pure iron. If the material to betreated has a high carbon content, the resulting carbonitride layercontains a correspondingly large amount of carbon. Therefore, a surfacelayer having a high carbon content can be formed if the carbon contentof the surface portion of the material to be treated is increased bycarburization or otherwise prior to, or during, nitriding. In case thematerial to be treated is industrial pure iron, the carbon which itcontains only in a very small quantity is the carbon which the resultingcarbonitride layer will contain.

The iron alloy material is, first, nitrided. The nitriding is thetreatment which causes the diffusion of nitrogen through the surface ofthe iron alloy material to form a nitride layer thereon. The nitridelayer is composed of iron nitride formed by the reaction of iron andnitrogen, or iron carbonitride formed by the reaction of iron, nitrogenand the carbon in the material. A layer of a solid solution of nitrogenin iron (diffusion layer) is formed immediately under the nitride layer.Then, the material is heated with a material containing a surfacelayer-forming element so that the element may be diffused through thenitride layer and replace the iron which it contains. If the nitridelayer is a layer of iron carbonitride, a layer composed of thecarbonitride of the surface layer-forming element is formed on thesurface of the iron alloy material. If it is a layer of iron nitride,the resulting surface layer is composed of the nitride of the surfacelayer-forming element. The maximum thickness of the surface layer whichcan be formed on the nitrided iron alloy material is equal to thethickness of the nitride layer. Therefore, the nitriding treatmentdefines the thickness of the surface layer which can be formed.

The nitride layer can be formed by any nitriding method, for example,gas nitriding or soft-nitriding, salt bath soft-nitriding or glowdischarge nitriding. It is preferable that the nitride layer have a highnitride concentration and a large thickness. The most preferablethickness of the nitride layer is from 3 to 15 microns. If the thicknessof the nitride layer is too small, the nitride or carbonitride of thesurface layer-forming element employed forms a layer having anunsatisfactorily small thickness. If the thickness of the nitride layeris too large, it is likely to lower the toughness of the iron alloymaterial.

After the iron alloy material has been nitrided, it is heated with amaterial containing one or more surface layer-forming elements, so thatthe element or elements may be diffused into the surface of the ironalloy material to form a layer of its nitride or carbonitride thereon.

The material containing one or more surface layer-forming elements isused to supply the element or elements which are diffused into thesurface of the iron alloy material. One or more elements are selectedfrom among chromium, Group Va elements (vanadium, niobium and tantalum),titanium and zirconium. The material containing one or more surfacelayer-forming elements may be, for example, a metal or compoundcontaining any such element.

The metals containing those elements are elemental metals, such as purechromium, metallic titanium and metallic zirconium, or alloys, such asferrochrome, ferrovanadium, ferrotitanium and ferrozirconium. Thecompounds containing those elements include chlorides, fluorides, oxidesand bromides, such as CrCl₃, VCl₃, NbCl₃, TiCl₃, ZrCl₄, CrF₆, K₂ NbF₇,NbF₅, VF₅, K₂ TaF₇, TiF₄, Na₂ TiF₆, Na₂ ZrF₆, Cr₂ O₃, K₂ CrO₃, V₂ O₅,NaVO₃, Nb₂ O₅, Ta₂ O₅, TiO₂, ZrO₂ and TiBr₄. One or more of these metalsand compounds are employed. It is the most practical choice to use oneor more metals, such as pure chromium, metallic titanium andferrozirconium.

When the iron alloy material is heated with the material or materialscontaining a surface layer-forming element, a treating agent is alsoused. It is used to assist the diffusion of the surface layer-formingelement into the surface of the iron alloy material. The agent may becomposed of at least one compound selected from among the chlorides,fluorides, borofluorides, oxides, bromides, iodides, carbonates,nitrates and borates of alkali or alkaline earth metals, or an ammoniumor metal halide or both, or a combination of at least one such alkali oralkaline earth metal compound with an ammonium or metal halide or both.An appropriate agent is selected to suit the method which is employedfor the heat treatment of the iron alloy material.

Specific examples of the alkali and alkaline earth metal compoundsinclude NaCl, CaCl₂, LiCl, NaF, KF, LiF, KBF₄, Na₂ CO₃, LiCO₃, KCO₃,NaNO₃ and Na₂ O. One or more of these compounds may be used. Theammonium halide may be selected from among, for example, NH₄ Cl, NH₄ Br,NH₄ I and NH₄ F, and the metal halide from among, for example, CrI₂,CrBr₃, TiF₄, VCl₃, FeCl₃ and TiBr₄. If the agent contains one or more ofthe surface layer-forming elements, the resulting surface layer may alsocontain the surface layer-forming element or elements which the agentcontains. Therefore, the agent which is composed of a halide of anysurface layer-forming element, such as CrCl₃, VCl₃, NbCl₃, TiF₄ orZrCl₄, may also serve as the material containing the surfacelayer-forming agent.

The salt bath immersion or electrolysis method and the powder packmethod are examples of the method which can be employed for the heattreatment of the iron alloy material. The choice of a particular methoddepends on whether the treating agent is in a molten or solid state atthe temperature at which the heat treatment is carried out. The powderpack method can further be classified into the pack, paste, non-contactand fluidized bed methods.

The salt bath immersion method is a method in which the iron alloymaterial and the material containing a surface layer-forming element areimmersed in a bath composed of the molten treating agent. The treatingagent which can be used when this method is employed may be composed ofat least one of the chlorides, fluorides, borofluorides, carbonates,nitrates, oxides and borides of the alkali or alkaline earth metals,and/or a metal halide which melts, but does not vaporize, at atemperature below that at which the heat treatment is carried out. Theuse of two or more of those compounds, e.g. NaCl and CaCl₂, is preferredto maintain a satisfactory molten state. An oxide, such as Al₂ O₃ orZrO₂, or a cyanide, such as NaCN, can be added for adjusting theviscosity of the bath, or for other purposes.

The material containing the surface layer-forming element is immersed inthe bath so that the element may be dissolved therein. When the materialis immersed, it may be in the form of powder, preferably having a grainsize not larger than 200 mesh, or in the form of a thin sheet, so thatthe surface layer-forming element may be allowed to dissolve.Alternatively, a rod or sheet of the material is immersed in the bathand used as an anode for the anodic dissolution of the element. This wayof dissolution enables the element to dissolve quickly and therebyachieves an improved efficiency of work. Moreover, no undissolvedmaterial settles at the bottom of the bath. A vessel for the bath or aconductive substance placed therein serves as a cathode. Although a highanode current density brings about a high speed of dissolution, it issufficient to employ a relatively low current density, insofar as theelement can be dissolved even if no electrolysis takes place. Apractically appropriate current density ranges from 0.1 to 0.8 A/cm².

The surface layer-forming element dissolved in the bath is diffusedthrough the surface of the nitride layer on the iron alloy material andforms a nitride or carbonitride layer thereon.

The vessel for the bath may be made of, for example, graphite or steel.A steel vessel is satisfactory for practical purposes.

The salt bath electrolysis method is a method in which the iron alloymaterial is used as a cathode. It is immersed in a bath composed of themolten treating agent and containing a surface layer-forming elementdissolved therein. A vessel for the bath or a conductive substanceplaced therein is used as an anode.

The treating agent used for the salt bath immersion method can be usedfor the salt bath electrolysis method, too. The surface layer-formingelement can be dissolved in the bath by using the methods as hereinabovedescribed in connection with the salt bath immersion method.Alternatively, it is also possible to dissolve the element byelectrolysis using the material containing it as an anode and the ironalloy material as a cathode. This alternative method has the advantageof accomplishing the anodic dissolution of the element and the formationof a surface layer simultaneously.

A cathode current density not exceeding 2 A/cm² is employed for carryingout the salt bath electrolysis method. A practically appropriate currentdensity ranges from 0.05 to 0.8 A/cm².

Both of the salt bath immersion and electrolysis methods may be carriedout in the open air or in a protective gas, such as nitrogen or argon.

The powder pack method is a method in which the iro alloy material isheated in the presence of a mixed powder composed of the powder of atreating agent and the powder of a material containing a surfacelayer-forming element. This method has a number of variations. The packmethod is a method characterized by packing the iron alloy material inthe mixed powder. The paste method employs a paste of the mixed powdercovering the surface of the iron alloy material. The non-contact methodis characterized by placing the iron alloy material in an appropriatelydefined space in such a way that it may not contact the mixed powder.According to the fluidized bed method, the iron alloy material is placedin a fluidized bed of the mixed powder.

The treating agent which can be used for any of th variations of thepowder pack method is composed of at least one of the chlorides,fluorides, bromides, iodides and borofluorides of alkali or alkalineearth metals, and/or an ammonium or metal halide or both. Referringparticularly to the fluidized bed method, the metal halide is a compoundwhich sublimes or vaporizes at a temperature not higher than thetemperature which is employed for the heat treatment of the iron alloymaterial, e.g., VCl₃, FeCl₃, TiF₄, VF₃ or TiBr₄. If a metal halide whichdoes not sublime or vaporize at such a temperature is used, the treatingagent fails to generate a sufficiently large amount of gas to cause thesatisfactory diffusion of the surface layer-forming element to form asurface layer having a satisfactory thickness.

The mixed powder preferably contains 0.5 to 20% by weight of thetreating agent based on the weight of the material containing thesurface layer-forming element. If the proportion of the treating agentdeviates from the range hereinabove specified, it is difficult to formcontinuously a surface layer composed of the nitride or carbonitride ofthe surface layer-forming element. On the other hand, it becomes easierto form a surface layer continuously, as the proportion approaches themean value of the range.

The mixed powder may be composed of particles having a particle sizeless than about 100 mesh when the pack, paste or non-contact method isused. When the fluidized bed method is used, it preferably has aparticle size of 60 to 350 mesh. A mixed powder having a particle sizewhich is coarser than 60 mesh requires a large amount of gas forfluidization and makes it difficult to form a surface layer smoothly. Amixed powder having a particle size which is finer than 350 mesh isdifficult to handle, as it floats easily. In case the pack, paste ornon-contact method is used, the use of coarser or finer particles doesnot produce any appreciably adverse result.

It is possible to add a certain substance or substances to the mixedpowder. For example, it is possible to add an adhesive selected from,for example, dextrins or water glasses, to the mixed powder which isused for the paste method. Some of the treating agents tend to solidifyduring the heat treatment. It is useful to add the powder of an inertsubstance such as alumina (Al₂ O₃), to a mixed powder containing anysuch treating agent. Some combinations of the material containing asurface layer-forming element and the treating agent may not be soeffective for forming a surface layer as the other combinations. Theaddition of a halide as an activator makes any of such combinations moreeffective. The amount of any such additive can be determined selectivelyon a case to case basis.

Referring in further detail to each variation of the powder pack method,the pack method employs a vessel holding the mixed powder. The ironalloy material to be treated is buried in the powder and the vessel isheated in an open or controlled atmosphere heating furnace so that theiron alloy material may be heated. A layer composed of the powder of aninert material, such as alumina, or a metal powder, such as iron-boronpowder, can be provided in the open end of the vessel for preventing airfrom entering it.

The paste method uses a paste which is prepared by adding an adhesive tothe mixed powder. The adhesive can be selected from among, for example,an aqueous dextrin solution, glycerin, water glass, ethylene glycol andalcohol. The paste is applied to the surface of the iron alloy materialto form thereon a layer having a thickness of usually at least 1 mm.Then, the iron alloy material is usually placed in a vessel and thevessel is heated in a furnace. This heating can be carried out in theopen air, but if it is carried out in a non-oxidizing atmosphere, it ispossible to employ a paste layer having a smaller thickness. If thepaste is applied to only a limited part of the surface of the iron alloymaterial, it is possible to form a surface layer covering only that partof the surface.

According to the non-contact method, the iron alloy material and themixed powder are placed in a closed space. The mixed powder is placedadjacent to the open end of the vessel to prevent air from entering it.The iron alloy material is so positioned in the vessel that it may notcontact with the mixed powder, and the vessel is heated. The absence ofcontact between the iron alloy material and the mixed powder providescertain operational advantages.

According to the fluidized bed method, the iron alloy material and themixed powder are placed in a fluidized bed furnace. The mixed powdercontains a refractory material, such as alumina, which prevents it fromforming a solid mass during its fluidization. A fluidizing gas isintroduced into the furnace to form a fluidized bed of the powder andthe furnace is heated. This method makes it possible to form a verysmooth surface layer having a uniform thickness, as the fluidized bedhas a uniform temperature distribution. The fluidizing gas may be aninert gas, such as argon, or a non-oxidizing gas, such as nitrogen. Thegas is supplied at a rate of preferably at least 50 cm per minute asmeasured in the fluidized bed, so that no powder may adhere to thesurface layer on the iron alloy material. It is advisable from thestandpoint of easy operation to employ a gas pressure of 0.5 to 2kg/cm².

The iron alloy material is heated at a temperature not exceeding 700° C.The use of any temperature exceeding 700° C. produces strain in the ironalloy material. The heating temperature is preferably not higher than580° C. if the surface layer-forming element is one of Group Va metals.The lower limit of the heating temperature is preferably 450° C. The useof a lower temperature results in the very slow formation of a surfacelayer. Therefore, it is generally advisable to employ a temperaturerange of 500° C. to 650° C. which coincides with the range oftemperatures used for the high temperature tempering of die steel or forthe tempering of strutural steel. If any of Group Va elements is used,it is practically appropriate to employ a temperature of 500° C. to 580°C.

The iron alloy material is heated for a period of time which usuallyranges from one to 50 hours, depending on the amount of the surfacelayer-forming element which the resulting surface layer should contain.If it is heated for a long time, the surface layer contains a largeamount of the surface layer-forming element.

It is practically appropriate to form a surface layer having a thicknessof, say, 3 to 15 microns.

According to this invention, a surface layer composed of the nitride orcarbonitride of a surface layer-forming element is formed on the ironalloy material. While the mechanism of its formation is not clear, thefollowing is a description of the mechanism which the inventors of thisinvention assume from the results of analysis by a microanalyzer and therelation which they have found between the length of heating time andthe thickness of the surface layer. The following description refers tothe formation of a surface layer composed of carbonitride. In thefollowing description, the suffixed letters "m", "n", "o" and "p" standfor numerals.

If the iron alloy material to be treated is nitrided, the nitrogen (N)supplied from an external source reacts with the iron (Fe) and carbon(C) in the surface portion of the iron alloy material to form a layer ofnitride which is expressed as Fe_(m) (C,N)_(n). A solid solution ofnitrogen, which is expressed as Fe-N, is also formed immediately underthe nitride layer.

Then, if the iron alloy material is heated with a material containing asurface layer-forming element (hereinafter expressed "M") and thetreating agent, the element M is diffused into the nitride layer. Thisdiffusion is a reaction which causes the substitution of M for Fe inFe_(m) (C,N)_(n) and thereby the formation of a layer of nitrideexpressed as (M,Fe)_(o) (C,N)_(p). If the nitride Fe_(m) (C,N)_(n) iscompletely converted to (M,Fe)_(o) (C,N)_(p), there is no further growthof the (M,Fe)_(o) (C,N)_(p) layer. The (M,Fe)_(o) (C,N)_(p) layercontains a larger amount of M and a smaller amount of Fe toward itssurface than toward the base material. Therefore, it is sometimes truethat it contains only a very small amount of Fe on or adjacent to itssurface and had better be expressed as a layer of M_(o) (C,N)_(p).

Thus, the thickness of the surface layer which is formed on the ironalloy material is equal to that of the nitride layer formed thereon bythe preceding nitriding treatment. Therefore, the conditions of thenitriding treatment dictate the maximum thickness of the surface layer.The surface layer existing on the iron alloy material until the nitrideFe_(m) (C,N)_(n) is completely converted to (M,Fe)_(o) (C,N)_(p), iscomposed of a layer of (M,Fe)_(o) (C,N)_(p) growing adjacent to itssurface and a layer of Fe_(m) (C,N)_(n) remaining adjacent to the basematerial. The thickness of this surface layer is substantially equal tothat of the initial Fe_(m) (C,N)_(n) layer.

It is assumed that the foregoing description is also applicable to theformation of a surface layer composed of the nitride of a surfacelayer-forming element.

The mechanism as hereinabove described is possible, since the method ofthis invention employs a low heating temperature not exceeding 700° C.There is not known any method of forming a surface layer in accordancewith any such mechanism and any such relationship between heating timeand layer thickness. Referring, for instance, to EXAMPLE 1, the heatingtime did not have any effect on the thickness of the surface layer, i.e.the total thickness of the Fe_(m) (C,N)_(n) layer and the (M,Fe)_(o)(C,N)_(p) layer, when a heating temperature of 550° C. was employed, asshown by a curve Al. On the other hand, when a temperature of 1000° C.was employed, the thickness of the surface layer increased with anincrease in heating time, as shown by a curve SL. This is exactly whatresults from ordinary diffusion treatment.

In practice, the initial nitride layer need not be completely convertedto (M,Fe)_(o) (C,N)_(p), but two layers can exist together. This is truewhen the initial nitride layer is composed solely of iron and nitrogen,too.

According to this invention, a surface layer-forming element is diffusedat a temperature not exceeding 700° C. in the presence of a particulartreating agent into the surface of an iron alloy material on which alayer of nitride composed of iron and nitrogen, or iron, carbon andnitrogen has been formed. It is possible to form an excellent surfacelayer composed of the nitride or carbonitride of the surfacelayer-forming element on the iron alloy material even at a lowtemperature.

The use of a low temperature substantially completely prevents thegrowth of strain in the iron alloy material. The use of a lowtemperature ensures the ease of operation and does not require a largeamount of energy.

According to this invention, the surface layer is formed by diffusion.Despite the use of a low temperature, therefore, the layer is dense andadheres firmly to the base material, as opposed to a carbide or nitridelayer formed by PVD without relying on any diffusion reaction. Moreover,the layer has a practically satisfactory thickness.

The method of this invention requires only a very short time for forminga surface layer, as compared with any known method of forming a surfacelayer of carbide without employing any preliminary nitriding treatment.

The invention will now be described more specifically with reference toa variety of examples. In the following examples, % is shown by weightunless otherwise noted.

EXAMPLE 1

A specimen in the form of a round bar of high speed tool steel (AISI M2)having a diameter of 6 mm and a length of 30 mm was nitrided byimmersion in a salt bath having a temperature of 570° C. for two hours.A vessel made of heat-resistant steel and holding a mixture composed of52 mol% of CaCl₂ and 48 mol % of NaCl was heated in an electric furnacein the air to form a molten salt bath having a temperature of 550° C. Apowder of pure chromium having a particle size under 100 mesh was addedto the bath. The amount of the powder added was 20% of the weight of thebath. The specimen was immersed in the salt bath having a temperature of550° C. After a period of one to 25 hours had passed, it was taken outand quenched in oil. After the bath material adhering to the specimenhad been washed away, an end surface thereof was ground to prepare asectional surface for the examination of a surface layer formed on thespecimen and the thickness of the layer was measured. The results areshown by a curve AL in FIG. 1. The thickness of the layer which is shownby the curve AL at the immersing time of 0 hour is that of the initiallyformed nitride layer. The thickness shown thereafter is the thickness ofthe whole surface layer, i.e., the total thickness of the remainingnitride layer and the growing layer of chromium carbonitride. Thethickness of the carbonitride layer per se is shown by a curve BL. Thethickness of the whole surface layer did not show any appreciable changewith the lapse of the immersing time, but was always about four microns.

FIG. 2 is a photomicrograph of 400 magnifications showing in profile thesurface layer which was formed after nine hours of immersion treatment.It was a layer having a smooth surface. The layer and the base materialhad therebetween a boundary having a complicated contour defining anintimate bond therebetween. The layer was analyzed by an X-raymicroanalyzer. The results are shown in FIG. 3. The analysis revealedthe presence of N and C, as well as Cr, in the layer. The layer wasfound to contain about 60% of chromium. The examination of the layer byX-ray diffraction gave diffraction patterns indicating the presence ofCr₂ N and CrN. Therefore, it was concluded that the layer was a combinedlayer of chromium carbonitrides expressed as (Cr,Fe)₂ (N,C) and(Cr,Fe)(N,C).

For the sake of comparison, an equally sized and nitrided sample of AISIM2 steel was treated by immersion in a salt bath of the same compositionheated to a temperature of 1000° C., whereby a layer of chromiumcarbonitride was formed on the sample. The thickness of this layershowed an increase with the immersing time, as shown by a curve SL inFIG. 1. As the layer formed in accordance with this invention did notshow any appreciable change in thickness with the immersing time, it isclear that the mechanism of its formation differed from that throughwhich the layer was formed on the comparative sample.

EXAMPLE 2

The procedure of EXAMPLE 1 was repeated for nitriding a round bar ofAISI 1045 structural steel having a diameter of 7 mm and a length of 50mm and preparing a molten salt bath composed of a mixture of CaCl₂ andNaCl. A powder of CrCl₃ having a particle size under 320 mesh was addedto the bath. The amount of the powder added was 15% of the weight of thebath. The bath was heated to a temperature of 500° C. and the specimenwas immersed therein. After a period of one to 16 hours had passed, thespecimen was taken out and quenched in oil.

A plurality of specimens were treated for different periods of time, ashereinabove described. All of the surface layers formed on the specimenswere found to be of almost the same thickness and structure irrespectiveof the length of immersing time. FIG. 4 is a photomicrograph of 400magnifications showing in profile the surface layer formed by four hoursof treatment. It had a thickness of about eight microns. The examinationof the layer by X-ray diffraction and the analysis thereof by an X-raymicroanalyzer indicated that it was a combined layer of chromiumcarbonitrides (Cr,Fe)₂ (N,C) and (Cr,Fe)(N,C). The results of theanalysis by an X-ray microanalyzer are shown in FIG. 5.

EXAMPLE 3

A cylindrical specimen of AISI 1048 structural steel having an outsidediameter of 10 mm, an inside diameter of 6 mm and a length of 25 mm wasgiven six hours of gas soft-nitriding treatment at a temperature of 570°C. The procedure of EXAMPLE 1 was repeated for preparing a molten saltbath composed of CaCl₂ and NaCl. A powder of Al₂ O₃ having a particlesize under 320 mesh and a powder of ferrochrome having a particle sizeunder 200 mesh were added to the bath. The amounts of Al₂ O₃ andferrochrome added were 3% and 20%, respectively, of the weight of thebath. The bath was heated to 550° C. and the specimen was immersedtherein. After a certain period of time had passed, it was taken out andquenched in oil.

Four specimens, which had been prepared as hereinabove described, weretreated for one, nine, 25 and 50 hours, respectively. Then, they wereexamined for roundness. All of them showed substantially the sameroundness with a deviation of only about five microns at both the upperand lower ends thereof.

For the sake of comparison, another specimen was immersed in the bathhaving a temperature of 850° C. and treated for four hours. It showed aroundness deviation of about 20 microns which was about four times worsethan the roundness of any of the specimens treated in accordance withthis invention.

One of the specimens treated in accordance with this invention, whichhad been treated at 550° C. for 50 hours, was cut in profile forstructure examination. FIG. 6 is a photomicrograph of 400 magnificationsshowing in profile the surface layer formed on that specimen. It wasalso analyzed by an X-ray microanalyzer. The results are shown in FIG.7. It was a combined layer of chromium carbonitrides (Cr,Fe)₂ (N.C) and(Cr,Fe)(N,C) having a thickness of about eight microns.

EXAMPLE 4

A specimen in the form of a round bar of AISI M2 high speed tool steelhaving a diameter of 6 mm and a length of 30 mm was ionically nitridedat 550° C. for three hours. A molten salt bath was prepared in agraphite vessel. Its composition was equal to that of the bath used inEXAMPLE 1. A plate of pure chromium having a length of 40 mm, a width of35 mm and a thickness of 4 mm was placed in the center of the bath. Itwas used as an anode, and the graphite vessel as a cathode. An electriccurrent was supplied for about 15 hours so that the anode might have acurrent density of 0.8 A/cm². The decrease in weight of the chromiumplate which had resulted from the anodic dissolution was measured tocalculate the amount of chromium dissolved in the bath. It was about 7%of the weight of the bath. The specimen was immersed in the bath at 550°C. for nine hours, taken out and quenched in oil.

Then, the specimen was cut in profile for examination by an X-raymicroanalyzer. The surface layer which had been formed on the specimenwas found to contain carbon, in addition to chromium and nitrogen, asshown in FIG. 8. The examination of the surface layer by X-raydiffraction gave diffraction patterns coinciding closely with those ofCrN and Cr₂ N. Thus, it was concluded as a chromium carbonitride layer.

EXAMPLE 5

A specimen in the form of a round bar of AISI 1045 structural steelhaving a diameter of about 7 mm and a length of 50 mm was nitrided in asalt bath at 570° C. for an hour. A graphite vessel holding a mixturecomposed of 50 mol % of KF and 50 mol % of LiF was heated to 600° C. inan electric furnace in the air to prepare a molten salt bath. A powderof pure chromium having a particle size under 100 mesh was added to thebath. The amount of the powder added was 25% of the weight of the bath.The specimen was immersed in the bath having a temperature of 600° C. Itwas used as a cathode, and the graphite vessel as an anode. An electriccurrent was supplied for eight hours so that the cathode might have acurrent density of 0.1 A/cm².

Then, the specimen was taken out and quenched in oil. The surface layerwhich had been formed by electrolysis was examined by an X-raymicroanalyzer. The results of the examination showed that it was a layercomposed of (Cr,Fe)₂ (C,N) and (Cr,Fe)(C,N). The analysis of the layerfrom its surface showed that it contained nitrogen and carbon, as wellas about 60% of chromium.

EXAMPLE 6

The procedure of EXAMPLE 1 was repeated for nitriding a specimen of AISIM2 steel. A vessel made of heat-resistant steel and holding a mixture of45% of Li₂ CO₃, 25% of K₂ CO₃ and 30% of Na₂ CO₃ was heated to 550° C.in an air atmosphere furnace to prepare a molten salt bath. A powder ofpure chromium having a particle size under 100 mesh mesh was added tothe bath. The amount of the powder added was 30% of the weight of thebath. After the bath had been fully stirred, the specimen was immersedtherein and held at a temperature of 550° C. for four hours, whereby asurface layer was formed on the specimen.

After the specimen had been taken out and quenched in oil, the surfacelayer was examined by an X-ray microanalyzer. It was a layer composed of(Cr,Fe)₂ (C,N) and (Cr,Fe)(C,N).

EXAMPLE 7

A specimen of AISI 1045 steel having a diameter of 8 mm and a length of30 mm was given 150 minutes of gas soft-nitriding treatment at 570° C.Then, it was packed in a mixed powder in a stainless steel vessel. Thepowder was composed of 90% of pure chromium and 10% of potassiumborofluoride (KBF₄) and had a particle size under 100 mesh A powder offerroboron having a particle size under 100 mesh was placed on the mixedpowder to form an antioxidizing layer having a thickness of 3 to 4 mm.The vessel was heated at 600° C. for 16 hours in an atmospheric furnace.After the vessel had been taken out of the furnace, it was allowed tocool in the air and the specimen was removed from the powder.

The surface layer which had been formed on the specimen was examined byan X-ray microanalyzer. It was a layer composed of chromium, nitrogenand carbon, as shown in FIG. 9. The analysis of the layer from itssurface indicated the presence of about 60 % of chromium. Thus, it was achromium carbonitride layer.

EXAMPLE 8

A specimen of AISI W1 carbon tool steel having a diameter of 7 mm and alength of 30 mm was given 60 hours of gas nitriding treatment at 570° C.Then, it was heated at 650° C. for 16 hours in a mixed powder which wasequal to that which had been used in EXAMPLE 7, whereby a surface layerwas formed on the specimen. It was examined by an X-ray microanalyzer.It was a layer composed of chromium, nitrogen and carbon, as shown inFIG. 10. The analysis of the layer from its surface showed the presenceof about 50% of chromium. Thus, it was a chromium carbonitride layer.

EXAMPLE 9

A paste was prepared from a mixed powder composed of 40% of alumina (Al₂O₃) having a particle size under 200 mesh mesh, 55% of ferrochromehaving a particle size under 100 mesh and 5% of ammonium chloride (NH₄Cl) having a particle size of 80 to 100 mesh by using a solvent whichhad been prepared by dissolving ethyl cellulose in ethyl alcohol. Theconditions of EXAMPLE 8 were repeated for gas nitriding a specimen ofAISI W1 carbon tool steel having a diameter of 20 mm and a length of 10mm. The paste was applied to the surface of the specimen to form a layerhaving a thickness of 3 to 5 mm thereon. Then, the specimen was placedin a stainless steel vessel and heated at 600° C. for 16 hours in anargon gas atmosphere, whereby a surface layer was formed thereon.

The examination of the surface layer by an X-ray microanalyzer showedthat it was a chromium carbonitride layer.

EXAMPLE 10

A mixed powder composed of 60% of Al₂ O₃ having a particle size under 80mesh, 38.8% of pure chromium having a particle size under 100 mesh and1.2% of NH₄ Cl having a particle size under 80 mesh was placed in afluidized bed furnace. Argon gas was introduced into the furnace throughits bottom to fluidize the powder. The argon gas had a pressure of 1.5kg/cm² at the inlet of the furnace and a flow rate of 200 cm/min. in thefurnace. A round bar of AISI W1 carbon tool steel having a diameter of 7mm and a length of 50 mm, which had been nitrided in a salt bath underthe conditions employed in EXAMPLE 1, was placed in the furnace andheated at 600° C. for 16 hours, whereby a surface layer was formed onthe bar.

The examination of the surface layer by an X-ray microanalyzer showedthat it was composed of chromium, nitrogen and carbon, as shown in FIG.11. The analysis of the layer from its surface showed the presence ofabout 40% of chromium. The examination of the layer by X-ray diffractiongave a diffraction pattern coinciding closely with that of CrN. Thus, itwas a layer of (Cr,Fe)(C,N).

EXAMPLE 11

A specimen of AISI H13 hot working die tool steel having a diameter of 7mm and a length of 50 mm was nitrided in a salt bath at 570° C. for fourhours. A mixed powder composed of 58.8% of Al₂ O₃ having a particle sizeunder 80 mesh, 40% of pure chromium having a particle size under 100mesh and 1.2% of NH₄ Cl having a particle size under 80 mesh was placedin a fluidized bed furnace. Argon gas was introduced into the furnacethrough its bottom at a pressure of 1.5 kg/cm² and a flow rate of 200cm/min. to fluidize the powder. The specimen was placed in the furnaceand heated at 600° C. for 16 hours, whereby a surface layer was formedthereon.

The examination of the surface layer by an X-ray microanalyzer indicatedthat it was composed of chromium, nitrogen and carbon, as shown in FIG.12. The analysis of the layer from its surface showed the presence ofabout 60% of chromium. The examination of the layer by X-ray diffractiongave a diffraction pattern coinciding closely with that of CrN. Thus, itwas a layer of (Cr,Fe)(C,N).

EXAMPLE 12

A specimen of AISI 1045 steel having a diameter of 6.5 mm and a lengthof 40 mm was nitrided in a salt bath under the conditions employed inEXAMPLE 1. A mixture composed of 52 mol % of CaCl₂ and 48 mol % of NaClwas placed in a vessel made of heat-resistant steel and heated to 600°C. in an atmospheric electric furnace to prepare a molten salt bath. Apowder of pure chromium having a particle size under 200 mesh was addedto the bath. The amount of the powder added was 25% of the weight of thebath. The specimen was immersed in the bath having a temperature of 600°C. and after eight hours had passed, it was taken out and quenched inoil, whereby a surface layer was formed thereon. The examination of thelayer by X-ray diffraction gave diffraction patterns corresponding tothose of (Cr,Fe)₂ (C,N) and (Cr,Fe)(C,N). Thus, it was a chromiumcarbonitride layer.

Then, a dry friction test was conducted on the specimen (Specimen No.Cl) by using a Falex lubricant testing machine. The test was conductedat a load of 200 kg, a rotating speed of 300 rpm and a friction rate of0.1 m/sec. by using a testing member formed from gas carburized SCM415[JIS(Japanese Industrial Standards)] steel chromium molybdenum steel)containing 0.12 to 0.18 of C, 0.85 to 1.25 of Cu and 0.15 to 0.35 of Mo.

For the sake of comparison, a friction test was conducted on a specimenof AISI 1045 steel which had not been nitrided or treated as hereinabovedescribed (Specimen No. S2), and a specimen of AISI 1045 steel which hadbeen nitrided, but had not been treated (Specimen No. S3).

Specimen No. S2 seized and showed a wear of about 90 mg/cm² when thetest was conducted for a period of only about three seconds. SpecimenNo. S3 showed a wear of about 35 mg/cm² when the test was conducted forthree minutes, though it did not seize. On the other hand, Specimen No.Cl according to this invention did not show any appreciable wear or anytrace of seizing when the test was conducted for three minutes.

The same friction test was conducted also on a specimen of AISI 1045steel having an approximately three-micron thick layer of vanadiumcarbide (VC) formed by three hours of immersion in a salt bath at atemperature of 900° C. and a specimen of AISI 1045 steel having anapproximately seven-micron thick layer of titanium carbonitride Ti(C,N)formed by four hours of chemical vapor deposition at 850° C. There wasno appreciable difference in wear between these specimens and SpecimenNo. Cl which had been treated in accordance with this invention.Therefore, it is evident that the surface layer which is formed by themethod of this invention is comparable in wear and seizing resistance tothe surface layer formed by the high temperature salt bath immersion orCVD method.

EXAMPLE 13

A plurality of specimens each in the form of a round bar of AISI M2steel having a diameter of 6 mm and a length of 30 mm were nitrided bytwo hours of immersion in a salt bath having a temperature of 570° C. Avessel made of heat-resistant steel and holding a mixture composed of 52mol % of CaCl₂ and 48 mol % of NaCl was heated in an atmosphericelectric furnace to form a salt bath having a temperature of 550° C. Apowder of ferrovanadium having a particle size under 100 mesh (Fe-Vcontaining 85% of vanadium) was added to the bath. The amount of thepowder added was 20% of the weight of the bath. Each of the nitridedspecimens was immersed in the bath and after a period of one to 25 hourshad passed, it was taken out and quenched in oil. After the bathmaterial adhering to each specimen had been washed away, it was groundto prepare a cross-sectional surface for structure examination and thethickness of the surface layer which had been formed on the specimen wasmeasured. The results are shown by a curve A2 in FIG. 13. The thicknesswhich is shown by curve A2 at the immersing time of 0 hour is that ofthe initially formed nitride layer. Each thickness that curve A2 showsthereafter, beginning at the immersing time of one hour, is thethickness of the whole surface layer, i.e., the total thickness of theremaining nitride layer and the growing vanadium carbonitride layer. Thethickness of the carbonitride layer per se is shown by curve B2. Thethickness of the whole surface layer did not show any appreciable changeirrespective of the immersing time, but was about four microns.

FIG. 14 is a photomicrograph of 400 magnifications showing in profilethe surface layer formed by eight hours of immersion treatment. It was alayer having a smooth surface. The layer and the base material hadtherebetween a boundary having a complicated contour forming an intimatebond therebetween. The examination of the layer by an X-raymicroanalyzer indicated that it contained nitrogen and carbon inaddition to vanadium, as shown in FIG. 15. The analysis of the layerfrom its surface showed that it contained about 45% of vanadium. Theexamination of the layer by X-ray diffraction gave a diffraction patterncorresponding to that of VN. Thus, it was a layer of vanadiumcarbonitride (V,Fe)(N,C).

For the sake of comparison, a plurality of specimens of AISI M2 steelwere nitrided under the same conditions and treated by immersion in asalt bath of the same composition heated to 1000° C. The surface layersof vanadium carbonitride formed on these specimens showed an increase inthickness with an increase in immersing time, as shown by curve S4 inFIG. 13. As such was not the case with the thickness of any of thelayers formed in accordance with this invention, it is evident that themechanism of its formation differed from the mechanism through which thelayers were formed on the comparative specimens which had been treatedat a higher temperature.

EXAMPLE 14

The procedures of EXAMPLE 13 were repeated for nitriding specimens ofAISI 1045 steel having a diameter of 7 mm and a length of 50 mm andpreparing a salt bath from CaCl₂ and NaCl. A powder of VCl₃ having aparticle size under 320 mesh was added to the bath. The amount of thepowder added was 15% of the weight of the bath. The bath was heated to500° C. and each specimen was immersed therein. After a period of one to16 hours had passed, the specimens were taken out and quenched in oil.

The surface layers which had been formed on the specimens were all ofvirtually the same thickness and structure. Referring by way of exampleto the specimen which was treated for four hours, FIG. 16 is aphotomicrograph of 400 magnifications showing a profile of the layerwhich was formed on its surface. It had a thickness of about eightmicrons. The examination of the layer by X-ray diffraction and by anX-ray microanalyzer showed that it was a layer of vanadium carbonitride(V,Fe)(N,C). The results of the examination by an X-ray microanalyzerare shown in FIG. 17.

EXAMPLE 15

Two specimens each in the form of a cylinder having an outside diameterof 10 mm, an inside diameter of 6 mm and a length of 25 mm and made ofAISI 1048 structural steel were given six hours of gas soft-nitridingtreatment at 570° C. A salt bath of the same composition as that used inEXAMPLE 13 was prepared from CaCl₂ and NaCl. A powder of Al₂ O₃ having aparticle size under 320 mesh and a powder of ferrovanadium containing85% of vanadium) having a particle size under 200 mesh were added to thebath. The amounts of the alumina and ferrovanadium added were 3% and20%, respectively, of the weight of the bath. The bath was heated to550° C. and the specimens were immersed therein. When nine hours hadpassed, one of the specimens was taken out and quenched in oil and when25 hours had passed, the other specimen was taken out and quenched inoil.

The specimens were examined for roundness. They were substantially ofthe same roundness and showed a deviation of only about five micronsfrom a true circle at both the upper and lower ends thereof. For thesake of comparison, a specimen which had been treated in a salt bathhaving a temperature of 850° C. for four hours showed a deviation ofabout 20 microns which was about four times greater than that of any ofthe specimens which had been treated in accordance with this invention.

One of the specimens which had been treated at 550° C. for nine hours inaccordance with this invention was cut in profile to prepare a sectionfor the examination of the surface layer which had been formed thereon.The surface layer had a thickness of about eight microns. Theexamination of the layer by an X-ray microanalyzer showed that it was alayer of vanadium carbonitride (V,Fe)(N,C).

EXAMPLE 16

A specimen of AISI D2 cold working die tool steel having a diameter of 6mm and a length of 30 mm was ionically nitrided at 550° C. for threehours. A salt bath of the same composition as that used in EXAMPLE 13was prepared from CaCl₂ and NaCl in a graphite vessel. A sheet of Fe-Vcontaining 85% of vanadium having a length of 40 mm, a width of 35 mmand a thickness of 4 mm was placed in the center of the bath. This sheetwas used as an anode, and the graphite vessel as a cathode. An electriccurrent was supplied for about 15 hours so that the anode might have acurrent density of 0.6 A/cm². The decrease in weight of the Fe-V sheetshowed that its anodic dissolution had resulted in the bath containingabout 6% of vanadium. The specimen was immersed in the bath at 550° C.and when nine hours had passed, it was taken out and quenched in oil.

Then, the specimen was cut in profile for examination by an X-raymicroanalyzer. The surface layer which had been formed thereon was foundto contain carbon in addition to vanadium and nitrogen, as shown in FIG.18. The examination of the layer by X-ray diffraction gave a diffractionpattern coinciding closely with that of VN. Thus, it was a layer ofvanadium carbonitride.

EXAMPLE 17

A specimen of AISI 1045 steel having a diameter of about 7 mm and alength of 50 mm was nitrided in a salt bath at 570° C. for an hour. Agraphite vessel holding a mixture composed of 50 mol % of KF and 50 mol% of LiF was heated to 580° C. in an atmospheric electric furnace toprepare a molten salt bath. A powder of Fe-V containing 85% of vanadiumhaving a particle size under 100 mesh was added to the bath. The amountof the powder added was 25% of the weight of the bath. The specimen wasimmersed in the bath. An electric current was supplied to carry outelectrolysis at a cathode current density of 0.05 A/cm² for eight hoursusing the specimen as a cathode and the graphite vessel as an anode.

Then, the specimen was taken out and quenched in oil. The surface layerwhich had been formed thereon was examined by an X-ray microanalyzer. Itwas a layer composed of (V,Fe)(C,N). The examination of the layer fromits surface showed that it contained nitrogen and carbon in addition toabout 45% of vanadium.

EXAMPLE 18

A specimen of industrial pure iron having a diameter of 8 mm and alength of 35 mm was ionically nitrided under the conditions employed inEXAMPLE 16. The pure ron was of the type not containing more than 0.03%of carbon. A salt bath of the same composition as that used in EXAMPLE13 was prepared from CaCl₂ and NaCl in a steel vessel. A powder of Fe-Vcontaining 85% of vanadium having a particle size of 200 mesh was addedto the bath. The amount of the powder added was 30% of the weight of thebath. The bath was heated to 580° C. and the specimen was immersedtherein. When eight hours had passed, it was taken out and quenched inoil.

FIG. 19 is a photomicrograph of 400 magnifications showing in profilethe surface layer which was formed on the specimen. It had a thicknessof about 12 microns which was substantially equal to the thickness ofthe ionically nitrided layer. The examination of the layer by an X-raymicroanalyzer showed that it contained nitrogen and carbon in additionto about 70% of vanadium.

The examination of the layer by X-ray diffraction gave diffractionpatterns indicating the presence of V₂ N and VN. Thus, it was a layercomposed of vanadium carbonitrides (V,Fe)₂ (C,N) and (V,Fe)(C,N).

EXAMPLE 19

The procedure of EXAMPLE 13 was repeated for nitriding a specimen ofAISI 1045 steel in a salt bath. A vessel made of heat-resistant steeland holding a mixture composed of 45% of Li₂ CO₃, 25% of K₂ CO₃ and 30%of Na₂ CO₃ was heated to 550° C. in an atmospheric electric furnace toprepare a molten salt bath. A powder of pure niobium having a particlesize of 100 mesh was added to the bath. The amount of the powder addedwas 30% of the weight of the bath. After the bath had been fullystirred, the specimen was immersed therein and held at 550° C. for fourhours, whereby a surface layer was formed on the specimen. Then, it wastaken out and quenched in oil.

The surface layer was examined by an X-ray microanalyzer. It was foundto contain nitrogen and carbon in addition to niobium, as shown in FIG.20. The examination of the layer by X-ray diffraction gave a diffractionpattern coinciding closely with that of NbN. Thus, it was a layercomposed of (Nb,Fe)(C,N).

EXAMPLE 20

A specimen of AISI W1 steel having a diameter of 8 mm and a length of 30mm was given 150 minutes of gas soft-nitriding treatment at 570° C. Itwas placed in a stainless steel vessel holding a mixed powder composedof 90% of Fe-V containing 85% of vanadium and 10% of potassiumborofluoride (KBF₄) and having a particle size under 100 mesh, so thatit might be buried in the powder. A powder of ferroboron having aparticle size under 100 mesh was placed on the mixed powder to form anantioxidizing layer having a thickness of 3 to 4 mm. The vessel washeated at 580° C. for 16 hours in an atmospheric furnace. The vessel wastaken out of the furnace and after it had been cooled by air, thespecimen was removed from the powder.

The surface layer which had been formed on the specimen was examined byan X-ray microanalyzer. It was found to contain vanadium, nitrogen andcarbon, as shown in FIG. 21. The examination of the layer from itssurface showed the presence of about 20% of vanadium. Thus, it was alayer of vanadium carbonitride.

EXAMPLE 21

A specimen of AISI 1045 steel having a diameter of 7 mm and a length of30 mm was given 60 hours of gas nitriding treatment at 570° C. Then, itwas placed in a mixed powder of the same composition as that used inEXAMPLE 20 and was heated at 580° C. for 16 hours, whereby a surfacelayer was formed on the specimen. The surface layer was examined by anX-ray microanalyzer. It was found to be composed of vanadium, nitrogenand carbon, as shown in FIG. 22. The examination of the layer from itssurface revealed the presence of about 40% of vanadium. Thus, it was alayer of vanadium carbonitride.

EXAMPLE 22

A paste was prepared from a mixed powder composed of 40% of Al₂ O₃having a particle size under 200 mesh, 55% of Fe-V containing 85% ofvanadium having a particle size under 100 mesh and 5% of NH₄ Cl having aparticle size of 80 to 100 mesh by using a solvent prepared bydissolving ethyl cellulose in ethyl alcohol. A specimen of AISI 1045steel having a diameter of 20 mm and a length of 10 mm was gas nitridedunder the conditions employed in EXAMPLE 21. The paste was applied tothe surface of the nitrided specimen to form a layer having a thicknessof 3 to 5 mm thereon. Then, the specimen was placed in a stainless steelvessel and heated at 580° C. for 16 hours in an argon gas atmosphere,whereby a surface layer was formed on the specimen.

The surface layer was examined by an X-ray microanalyzer. It was a layerof vanadium carbonitride.

EXAMPLE 23

A mixed powder composed of 60% of Al₂ O₃ having a particle size under 80mesh, 38.8% of Fe-V having a particle size under 100 mesh and 1.2% ofVCl₃ having a particle size under 80 mesh was placed in a fluidized bedfurnace. Argon gas was introduced into the furnace through its bottom tofluidize the powder. The argon gas had a flow rate of 200 cm/min. in thefurnace and a pressure of 1.5 kg/cm² when entering the furnace. A barspecimen of AISI W1 steel having a diameter of 7 mm and a length of 50mm was nitrided in a salt bath under the conditions employed in EXAMPLE13. Then, the specimen was placed in the furnace and heated at 580° C.for eight hours, whereby a surface layer was formed thereon.

The surface layer was examined by an X-ray microanalyzer. It was foundto be composed of vanadium, nitrogen and carbon, as shown in FIG. 23.The analysis of the layer from its surface revealed the presence ofabout 30% of vanadium therein. The examination of the layer by X-raydiffraction gave a diffraction pattern of VN. Thus, it was a layer of(V,Fe)(C,N).

EXAMPLE 24

A specimen of AISI W1 steel having a diameter of 7 mm and a length of 50mm was nitrided in a salt bath at 570° C. for four hours. A mixed powdercomposed of 58.8% of Al₂ O₃ having a particle size under 80 mesh, 40% ofFe-Nb having a particle size under 100 mesh and 1.2% of NH₄ Cl having aparticle size under 80 mesh was placed in a fluidized bed furnace. Argongas was introduced into the furnace through its bottom to fluidized thepowder. The argon gas had a pressure of 1.5 kg/cm² when entering thefurnace and was caused to flow therethrough at a rate of 200 cm/min.Then, the specimen was placed in the furnace and heated at 580° C. for16 hours, whereby a surface layer was formed thereon.

The surface layer was examined by an X-ray microanalyzer. It was foundto be composed of niobium, nitrogen and carbon, as shown in FIG. 24. Theanalysis of the layer from its surface revealed the presence of about20% of niobium therein. The examination of the layer by X-raydiffraction gave a diffraction pattern of NbN. Thus, it was a layer of(Nb,Fe)(C,N).

EXAMPLE 25

A specimen of AISI M2 steel in the form of a sheet having a length of 60mm, a width of 20 mm and a thickness of 10 mm was nitrided in a saltbath under the conditions employed in EXAMPLE 1. A mixture composed of52 mol % of CaCl₂ and 48 mol % of NaCl was placed in a vessel made ofheat-resistant steel and was heated to 570° C. in an atmosphericelectric furnace to prepare a molten salt bath. A powder of Fe-Vcontaining 85% of vanadium having a particle size under 200 mesh wasadded to the bath. The amount of the powder added was 25% of the weightof the bath. The specimen was immersed in the bath at 570° C. and wheneight hours had passed, it was taken out and quenched in oil.

The surface layer which had been formed on the specimen was examined byX-ray diffraction. It gave a diffraction pattern of VN. Thus, it was alayer of vanadium carbonitride.

Then, the specimen was subjected to a dry wear test by means of anOgoshi's fast wear testing machine employing a testing member formedfrom spheroidizing annealed SCM415 (JIS) steel. The test was conductedat a final load of 3.3 kg, a slip distance of 600 m and a slip rate of 2m/sec. The specimen will hereinafter be referred to as Specimen No. C2.

For the sake of comparison, the same test was conducted on a specimen ofAISI M2 steel which had not been nitrided or treated as hereinabovedescribed (Specimen No. S5) and a specimen of AISI M2 steel which hadbeen nitrided, but had not been treated (Specimen No. S6)

The results of the tests are shown in a table below. As is obvious fromthe table, the surface layer which had been formed on Specimen No. C2showed a higher degree of wear resistance than the surfaces of SpecimensNos. S5 and S6.

                  TABLE                                                           ______________________________________                                                  Wear (mm.sup.3 /kg-m)                                               Specimen        C2         S5      S6                                         ______________________________________                                         Slip rate                                                                              2     m/sec.  1.3       8    5.5                                              4.4   m/sec.  0.8      18    16                                                   Invention                                                                              Comparative                                            ______________________________________                                    

The same wear test was also conducted on a specimen of AISI M2 steelhaving a approximately four-micron thick layer of vanadium carbide (VC)formed by two hours of immersion in a salt bath having a temperature of1000° C. and on a specimen of AISI M2 steel having an approximatelyseven-micron thick layer of titanium carbonitride TI (C,N) formed byfour hours of CVD at 850° C. There was no appreciable difference in wearbetween these specimens and Specimen No. C2. Therefore, it is evidentthat this invention makes it possible to form a surface layer which iscomparable in wear and seizing resistance to the surface layer formed bythe high temperature salt bath immersion or CVD method.

EXAMPLE 26

A plurality of specimens each in the form of a round bar of AISI M2steel having a diameter of 6 mm and a length of 30 mm were nitrided in asalt bath having a tcmperature of 570° C. for an hour. A vessel made ofheat-resistant steel and holding a mixture composed of 52 mol % of CaCl₂and 48 mol % of NaCl was heated in an atmospheric electric furnace toprepare a molten salt bath having a temperature of 550° C. A powder ofmetallic titanium having a particle size under 100 mesh was added to thebath. The amount of the powder added was 20% of the weight of the bath.The specimens were immersed in the bath and when different periods oftime ranging from one to 16 hours had passed, they were taken out andquenched in oil. After the bath material adhering to the specimens hadbeen washed away, a section of each specimen was ground to prepare across-sectional surface for use in structure determination and thethickness of the surface layer which had been formed on each specimenwas measured. The results are shown by curve A3 in FIG. 25. Thethickness of the layer shown by curve A3 at the immersing time of 0 houris the thickness of the initially formed nitride layer. The thicknesswhich is shown thereafter, beginning at the immersing time of 1 hour, isthe thickness of the whole surface layer, i.e., the total thickness ofthe remaining nitride layer and the growing titanium carbonitride layer.The thickness of the carbonitride layer per se is shown by curve B3. Thethickness of the whole surface layer did not show any appreciable changeirrespective of the immersing time, but was about three microns on allof the specimens.

FIG. 26 is a photomicrograph of 400 magnifications showing in profilethe surface layer which was formed on the specimen treated for ninehours. It was a layer having a smooth surface. The layer and the basematerial had therebetween a boundary having a complicated contourdefining an intimate bond therebetween. The examination of the layer byan X-ray microanalyzer revealed the presence of nitrogen and carbon inaddition to titanium, as shown in FIG. 27. The examination of the layerby X-ray diffraction gave a diffraction pattern of TiN. Thus, it was alayer of titanium carbonitride (Ti,Fe)(N,C).

For the sake of comparison, a number of specimens of AISI M2 steel werenitrided under the same conditions and treated in a salt bath of thesame composition which had been heated to 1000° C., whereby a layer oftitanium carbonitride was formed on the surface of each specimen. Thethickness of the layer is shown by curve S7 in FIG. 25. The thicknessincreased with an increase in immersing time. As such was not the casewith the surface layer formed in accordance with this invention, it isevident that the mechanism of its formation differed from the mechanismthrough which the layers were formed on the comparative specimensemploying a higher temperature.

EXAMPLE 27

A plurality of specimens of AISI 1045 steel each having a diameter of 7mm and a length of 50 mm were nitrided in a salt bath under theconditions employed in EXAMPLE 26. A molten salt bath of the samecomposition as that used in EXAMPLE 26 was prepared from CaCl₂ and NaCl.A powder of TiCl₃ having a particle size of 320 mesh was added to thebath. The amount of the powder added was 15% of the weight of the bath.The bath was heated to 500° C. and the specimens were immersed therein.When different periods of time ranging from one to 16 hours had passed,they were taken out and quenched in oil.

The surface layers which had been formed on the specimens were all ofsubstantially the same thickness and structure, irrespective of theimmersing time. Referring by way of example to the specimen which wastreated for six hours, FIG. 28 is a photomicrograph of 400magnifications showing a profile of the surface layer formed thereon. Ithad a thickness of about five microns. The examination of the layer byX-ray diffraction and by an X-ray microanalyzer showed that it was alayer of titanium carbonitride (Ti,Fe)(N,C). The results of theexamination by the X-ray microanalyzer are shown in FIG. 29.

EXAMPLE 28

A plurality of cylindrical specimens of AISI 1048 steel each having anoutside diameter of 10 mm, an inside diameter of 6 mm and a length of 25mm were given five hours of gas soft-nitriding treatment at 570° C. Amolten salt bath of the same composition as that used in EXAMPLE 1 wasprepared from CaCl₂ and NaCl. A powder of Al₂ O₃ having a particle sizeunder 320 mesh and a powder of ferrotitanium containing 45% of titaniumhaving a particle size under 200 mesh were added to the bath. Theamounts of the alumina and ferrotitanium added were 3% and 25%,respectively, of the weight of the bath. The bath was heated to 550° C.and the specimens were immersed therein. When nine hours had passed,they were taken out and quenched in oil.

The specimens were examined for roundness. They were substantially ofthe same roundness and showed a roundness deviation of only about sixmicrons at both the unner and lower ends thereof. For the sake ofcomparison, a specimen which had been immersed in a salt bath having atemperature of 850° C. for four hours showed a roundness deviation ofabout 25 microns which was about four times greater than that of any ofthe specimens which had been treated in accordance with this invention.

The specimens which had been treated at 550° C. for nine hours inaccordance with this invention were cut to expose the surface layerwhich had been formed thereon. It had a thickness of about five microns.The examination of the layer by X-ray diffraction and by an X-raymicroanalyzer showed that it was a layer of titanium carbonitride(Ti,Fe)(N,C). The analysis of the layer from its surface by an X-raymicroanalyzer revealed that it contained nitrogen and carbon in additionto about 40% of titanium.

EXAMPLE 29

A specimen of AISI M2 steel having a diameter of 6 mm and a length of 30mm was ionically nitrided at 550° C. for five hours. A molten salt bathof the same composition as that used in EXAMPLE 26 was prepared fromCaCl₂ and NaCl in a graphite vessel. A bar of metallic titanium having adiameter of 10 mm and a length of 50 mm was placed in the center of thebath. The bar was used as an anode, and the graphite vessel as acathode. An electric current was supplied to the anode at a density of0.5 A/cm² for about 16 hours. The decrease in weight of the titanium barshowed that the anodic dissolution of titanium had resulted in the bathcontaining about 10% of titanium.

The specimen was immersed in the bath at 550° C. and when eight hourshad passed, it was taken out and quenched in oil. The specimen was cutfor examination by an X-ray microanalyzer. The surface layer which hadbeen formed thereon was a layer of titanium carbonitride.

EXAMPLE 30

A specimen of AISI 1045 steel having a diameter of about 7 mm and alength of 50 mm was nitrided in a salt bath having a temperature of 570°C. for an hour. A graphite vessel holding a mixture composed of 50 mol %of KF and 50 mol % of LiF was heated to 600° C. in an air atmosphericelectric furnace to prepare a molten salt bath. A powder of metallictitanium having a particle size under 100 mesh was added to the bath.The amount of the powder added was 25% of the weight of the bath. Thespecimen was immersed in the bath. An electric current was supplied tocarry out electrolysis at a cathode current density of 0.08 A/cm² foreight hours using the specimen as a cathode and the graphite vessel asan anode, whereby a surface layer was formed on the specimen.

The specimen was taken out and quenched in oil. Then, the surface layerwas examined by an X-ray microanalyzer. It was a layer of (Ti,Fe)(C,N).The analysis of the layer from its surface showed that it containednitrogen and carbon in addition to about 50% of titanium.

EXAMPLE 31

A specimen of industrial pure iron having a diameter of 8 mm and alength of 35 mm was ionically nitrided under the conditions employed inEXAMPLE 29. The pure iron did not contain more than 0.03% of carbon. Amolten salt bath of the same composition as that used in EXAMPLE 26 wasprepared from CaCl₂ and NaCl in a steel vessel. A powder of Fe-Ticontaining 45% of titanium having a particle size under 200 mesh wasadded to the bath. The amounts of the powder added was 30% of the weightof the bath. The bath was heated to 600° C. and the specimen wasimmersed therein. After eight hours had passed, it was taken out andquenched in oil.

FIG. 30 is a photomicrograph of 400 magnifications showing in profilethe surface layer which was formed on the specimen. It had a thicknessof about 10 microns which was substantially equal to the thickness ofthe ionically nitrided layer. The analysis of the layer from its surfaceby an X-ray microanalyzer showed that it contained nitrogen and carbonin addition to about 50% of titanium. Thus, it was a layer of titaniumcarbonitride.

EXAMPLE 32

A specimen of AISI M2 steel was nitrided in a salt bath under theconditions employed in EXAMPLE 1. A vessel made of heat-resistant steeland holding a mixture composed of 45% of Li₂ CO₃, 25% of K₂ CO₃ and 30%of Na₂ CO₃ was heated to 550° C. in an air atmospheric furnace toprepare a molten salt bath. A powder of metallic titanium having aparticle size under 100 mesh was added to the bath. The amount of thepowder added was 30% of the weight of the bath. The bath was fullystirred and the specimen was immersed therein. After five hours hadpassed, it was taken out and quenched in oil.

The surface layer which had been formed on the specimen was examined byan X-ray microanalyzer. It was a layer of (Ti,Fe)(C,N).

EXAMPLE 33

A specimen of AISI 1045 steel having a diameter of 8 mm and a length of30 mm was given 150 minutes of gas soft-nitriding treatment at 570° C.Then, it was placed in a stainless steel vessel holding a mixed powdercomposed of 90% of Fe-Ti containing 45% of titanium having a particlesize under 100 mesh and 10% of potassium borofluoride (KBF₄) having aparticle size under 100 mesh, so that the specimen might be buried inthe powder. A power of ferroboron having a particle size under 100 meshwas placed on the mixed powder to form an antioxidizing layer having athickness of 3 to 4 mm. The vessel was heated at 600° C. for 16 hours inan atmospheric furnace. Then, the vessel was taken out of the furnaceand after it had been cooled by air, the specimen was removed from thepowder.

The surface layer which had been formed on the specimen was examined byan X-ray microanalyzer. It was a layer composed of titanium, nitrogenand carbon. The analysis of the layer from its surface showed that itcontained about 35% of titanium. Thus, it was a layer of titaniumcarbonitride.

EXAMPLE 34

A paste was prepared from a mixed powder composed of 40% of Al₂ O₃having a particle size under 200 mesh, 55% of Fe-Ti containing 45% oftitanium having a particle size under 100 mesh and 5% of NH₄ Cl having aparticle size under 80 mesh by using a solvent prepared by dissolvingethyl cellulose in ethyl alcohol. A specimen of AISI W1 steel having adiameter of 20 mm and a length of 10 mm was given gas soft-nitridingtreatment under the conditions employed in EXAMPLE 33. The paste wasapplied to the surface of the specimen to form a layer having athickness of 3 to 5 mm thereon. Then, the specimen was placed in astainless steel vessel and heated at 600° C. for 16 hours in an argongas atmosphere, whereby a surface layer was formed on the specimen.

The surface layer was examined by an X-ray microanalyzer. It was a layerof titanium carbonitride.

EXAMPLE 35

A mixed powder composed of 60% of Al₂ O₃ having a particle size under 80mesh, 38.8% of Fe-Ti containing 45% of titanium having a particle sizeunder 100 mesh and 1.2% of NH₄ Cl having a particle size under 80 meshwas placed in a fluidized bed furnace. Argon gas was introduced into thefurnace through its bottom to fluidize the powder. The argon gas had apressure of 5 kg/cm² when entering the furnace and was caused to flowtherethrough at a rate of 200 cm/min. A specimen of AISI H13 steelhaving a diameter of 7 mm and a length of 50 mm, which had been nitridedin a salt bath under the conditions employed in EXAMPLE 26, was placedin the furnace and heated at 600° C. for 16 hours, whereby a surfacelayer was formed on the specimen.

The surface layer was examined by an X-ray microanalyzer. It was a layercomposed of titanium, nitrogen and carbon, as shown in FIG. 31. Thesurface analysis of the layer showed that it contained about 30% oftitanium. The examination of the layer by X-ray diffraction gave adiffraction pattern of TiN. Thus, it was a layer of (Ti,Fe)(C,N).

EXAMPLE 36

A specimen of AISI W1 steel having a diameter of 7 mm and a length of 50mm was nitrided in a salt bath having a temperature of 570° C. for fourhours. A mixed powder composed of 58.8% of Al₂ O₃ having a particle sizeunder 80 mesh, 40% of Fe-Ti containing 45% of titanium having a particlesize of 100 to 200 mesh and 1.2% of NH₄ Cl having a particle size under80 mesh was placed in a fluidized bed furnace. Argon gas was introducedinto the furnace through its bottom to fluidize the powder. The argongas had a pressure of 1.5 kg/cm² when entering the furnace and wascaused to flow therethrough at a rate of 200 cm/min. The specimen wasplaced in the furnace and heated at 600° C. for 16 hours, whereby asurface layer was formed thereon.

The surface layer was examined by an X-ray microanalyzer. It was a layercomposed of titanium, nitrogen and carbon. The surface analysis of thelayer showed that it contained about 20% of titanium. The examination ofthe layer by X-ray diffraction gave a diffraction pattern of TiN. Thus,it was a layer of (Ti,Fe)(C,N).

EXAMPLE 37

A specimen of AISI M2 steel having a diameter of 6.5 mm and a length of40 mm was nitrided in a salt bath under the conditions employed inEXAMPLE 26. A mixture composed of 52 mol % of CaCl₂ and 48 mol % of NaClwas placed in a vessel made of heat-resistant steel and was heated to550° C. in an atmospheric electric furnace to prepare a molten saltbath. A powder of Fe-Ti containing 45% of titanium having a particlesize under 200 mesh was added to the bath. The amount of the powderadded was 25% of the weight of the bath. The specimen was immersed inthe bath and heated at 550° C. for eight hours, whereby a surface layerwas formed thereon. Then, it was taken out and quenched in oil.

The examination of the surface layer by X-ray diffraction gave adiffraction pattern of TiN. Thus, it was a layer of titaniumcarbonitride. This specimen will hereinafter be referred to as SpecimenNo. C3.

A dry friction test was conducted on Specimen No. C3 by means of a Falexlubricant testing machine employing a testing member formed from gascarburized SCM415 (JIS) steel. The test was conducted at a load of 400kg, a rotating speed of 300 rpm and a friction rate of 0.1 m/sec.

For the sake of comparison, the same friction test was conducted on aspecimen of AISI M2 steel which had not been nitrided or treated(Specimen No. S8) and a specimen of AISI M2 steel which had beennitrided, but had not been treated (Specimen No. S9).

The results of the tests are shown in a table below. Both of SpecimensNos. S8 and S9 showed clear marks of seizing, but Specimen No. C3 wasonly negligibly damaged. Therefore, it is evident that this inventioncan form a surface layer which is superior in wear and seizingresistance to the surface of any material such as Specimen No. S8 or S9.

                  TABLE                                                           ______________________________________                                        Specimen No.                                                                              C3            S8      S9                                          ______________________________________                                        Wear (mg/cm.sup.2)                                                                        4             12      9                                                     Invention   Comparative                                             ______________________________________                                    

EXAMPLE 38

A plurality of specimens each in the form of a round bar of AISI M2steel having a diameter of 6 mm and a length of 30 mm were nitrided in asalt bath having a temperature of 570° C. for three hours. A mixturecomposed of 52 mol % of CaCl₂ and 48 mol % of NaCl was placed in avessel made of heat-resistant steel and was heated to 550° C. in anatmospheric electric furnace to prepare a molten salt bath. A powder offerrozirconium (Fe-Zr) containing 80% of zirconium, which had a particlesize under 100 mesh, was added to the bath. The amount of the powderadded was 25% of the weight of the bath. The specimens were immersed inthe bath and after they had been held at 550° C. therein for differentperiods of time ranging from one to 25 hours, they were taken out andquenched in oil. After the bath material adhering to the specimens hadbeen washed away, a section of each specimen was ground to prepare across-sectional surface for use in structure determination and thethickness of the layer which had been formed on each specimen wasmeasured. The results are shown by curve A4 in FIG. 32. The thickness ofthe layer which is shown by curve A4 at the immersing time of 0 hour isthat of the initially formed nitride layer. The thickness which is shownthereafter, beginning at the immersing time of 1 hour, is the thicknessof the whole surface layer, i.e., the total thickness of the remainingnitride layer and the growing layer of zirconium carbonitride. Thethickness of the carbonitride layer per se is shown by curve B4. Thethickness of the surface layer did not show any appreciable changeirrespective of the immersing time, but was about five microns on allthe specimens.

FIG. 33 is a photomicrograph of 400 magnifications showing in profilethe surface layer which was formed by nine hours of immersion. It was alayer having a smooth surface. The layer and the base material hadtherebetween a boundary having a complicated contour forming an intimatebond therebetween. The examination of the layer by an X-raymicroanalyzer indicated that it contained nitrogen and carbon inaddition to zirconium, as shown in FIG. 34. The surface analysis of thelayer showed that it contained about 50% of zirconium. The examinationof the layer by X-ray diffraction gave a diffraction pattern of ZrN.Thus, it was a layer of zirconium carbonitride (Zr,Fe)(N,C).

For the sake of comparison, a plurality of specimens of AISI M2 steelwere nitrided under the same compositions and immersed in a salt bath ofthe same composition heated to 1000° C., whereby a layer of zirconiumcarbonitride was formed on the surface of each specimen. The surfacelayers on these specimens had a thickness increasing with an increase inimmersing time, as shown by curve S10 in FIG. 32. As such was not thecase with the surface layers formed in accordance with this invention(see curve A4), it is evident that the mechanism of their formationdiffered from the mechanism through which the surface layers were formedon the comparative specimens which had been heated at a highertemperature.

EXAMPLE 39

A plurality of specimens of AISI 1045 steel each having a diameter of 7mm and a length of 50 mm were nitrided in a salt bath under theconditions employed in EXAMPLE 38. A molten salt bath of the compositionused in EXAMPLE 38 was prepared from CaCl₂ and NaCl. A powder of ZrCl₄having a particle size under 320 mesh was added to the bath. The amountof the powder added was 15% of the weight of the bath. The bath washeated to 500° C. and the specimens were immersed therein for differentperiods of time ranging from one to 16 hours, whereby a surface layerwas formed on each specimen. Then, each specimen was taken out andquenched in oil.

The surface layers were all of substantially the same thickness andstructure irrespective of the immersing time. Referring by way ofexample to the specimen treated by four hours of immersion, FIG. 35 is aphotomicrograph of 400 magnifications showing a profile of the surfacelayer formed thereon. It had a thickness of about six microns. It wasexamined by X-ray diffraction and by an X-ray microanalyzer. The resultsof the examination by the X-ray microanalyzer are shown in FIG. 36. Itwas a layer of zirconium carbonitride (Zr,Fe)(N,C).

EXAMPLE 40

A plurality of cylindrical specimens of AISI 1048 steel each having anoutside diameter of 10 mm, an inside diameter of 6 mm and a length of 25mm were given six hours of gas soft-nitriding treatment at 570° C. Amolten salt bath of the composition used in EXAMPLE 38 was prepared fromCaCl₂ and NaCl. A powder of Al₂ O₃ having a particle size under 320 meshand a powder of Fe-Zr having a particle size under 200 mesh were addedto the bath. The amounts of the alumina and ferrozirconium added were 3%and 30%, respectively, of the weight of the bath. The ferrozirconium hada zirconium content of 80%. Then, the bath was heated to 550° C. and thespecimens were immersed therein. They were held in the bath for threedifferent periods of time, i.e., 1, 9 and 25 hours, respectively. Then,they were taken out and quenched in oil.

Then, the specimens were examined for roundness. They were allsubstantially of the same roundness and showed a roundness deviation ofonly about five microns at both the upper and lower ends thereof. Forthe sake of comparison, a specimen which had been immersed in a bathhaving a temperature of 850° C. for four hours showed a roundnessdeviation of about 20 microns which was about four times greater thanthat of any of the specimens which had been treated in accordance withthis invention.

One of the specimens treated in accordance with this invention, whichhad been treated at 550° C. for nine hours, was cut in profile and itssurface layer was examined. It had a thickness of about eight microns.The analysis of the layer by an X-ray microanalyzer showed that it was alayer of zirconium carbonitride (Zr,Fe)(N,C).

EXAMPLE 41

A specimen of AISI M2 steel having a diameter of 6 mm and a length of 30mm was ionically nitrided at 550° C. for four hours. A molten salt bathof the composition used in EXAMPLE 38 was prepared from CaCl₂ and NaClin a graphite vessel. A sheet of metallic zirconium having a length of40 mm, a width of 35 mm and a thickness of 4 mm was placed in the centerof the bath. This sheet was used as an anode, and the graphite vessel asa cathode. An electric current was supplied to the anode at a density of0.7 A/cm² for about 20 hours, whereby zirconium was anodically dissolvedin the bath. The resulting decrease in weight of the zirconium sheettaught that the bath contained about 6% of zirconium dissolved therein.The specimen was immersed in the bath and heated at 550° C. for eighthours. Then, it was taken out and quenched in oil.

Then, the specimen was cut in profile and the surface layer which hadbeen formed thereon was examined by an X-ray microanalyzer. It was foundto contain carbon, as well as zirconium and nitrogen. The examination ofthe layer by X-ray diffraction gave a diffraction pattern coincidingclosely with that of ZrN. Thus, it was a layer of zirconiumcarbonitride.

EXAMPLE 42

A specimen of AISI 1045 steel having a diameter of about 7 mm and alength of 50 mm was nitrided in a salt bath having a temperature of 570°C. for an hour. A mixture composed of 50 mol % of KF and 50 mol % of LiFwas placed in a graphite vessel and heated to 600° C. in an atmosphericelectric furnace to prepar a molten salt bath. A powder of Fe-Zrcontaining 80% of zirconium, which had a particle size under 100 mesh,was added to the bath. The amount of the powder added was 30% of theweight of the bath. The specimen was immersed in the bath as a cathode,while the graphite vessel was used as an anode. An electric current wassupplied to carry out electrolysis at a cathode current density of 0.05A/cm² for eight hours, whereby a surface layer was formed on thespecimen. Then, it was taken out and quenched in oil.

The surface layer was examined by an X-ray microanalyzer. It was a layerof (Zr,Fe)(C,N). The surface analysis of the layer showed that itcontained nitrogen and carbon in addition to about 65% of zirconium.

EXAMPLE 43

A specimen of industrial pure iron having a diameter of 8 mm and alength of 35 mm was ionically nitrided under the conditions employed inEXAMPLE 41. The iron had a carbon content not exceeding 0.03%. A moltensalt bath of the composition used in EXAMPLE 38 was prepared from CaCl₂and NaCl in a steel vessel. A powder of Fe-Zr containing 80% ofzirconium, which had a particle size of 200 mesh, was added to the bath.The amount of the powder added was 30% of the weight of the bath. Thebath was heated to 600° C. and the specimen was immersed therein. Wheneight hours had passed, it was taken out and quenched in oil.

FIG. 37 is a photomicrograph of 400 magnifications showing a profile ofthe surface layer which was formed on the specimen. It had a thicknessof about 12 microns which was substantially equal to the thickness ofthe initially formed nitride layer. The examination of the layer by anX-ray microanalyzer showed that it contained nitrogen and carbon inaddition to about 50% of zirconium. Thus, it was a layer of zirconiumcarbonitride.

EXAMPLE 44

A specimen of AISI D2 steel was nitrided in a salt bath under theconditions employed in EXAMPLE 38. A mixture composed of 45% of Li₂ CO₃,25% of K₂ CO₃ and 30% of Na₂ CO₃ was placed in a vessel made ofheat-resistant steel and was heated to 520° C. in a controlledatmosphere furnace to prepare a molten salt bath. A powder of metalliczirconium having a particle size under 200 mesh was added to the bath.The amount of the powder added was 30% of the weight of the bath. Afterthe bath had been fully stirred, the specimen was immersed therein andheated at 520° C. for five hours, whereby a surface layer was formed onthe specimen. Then, it was taken out and quenched in oil.

The surface layer was examined by an X-ray microanalyzer. It was a layerof (Zr,Fe)(C,N). FIG. 38 is a photomicrograph of 400 magnificationsshowing a profile of the surface layer. It had a thickness of about fivemicrons which was substantially equal to the thickness of the initiallyformed nitride layer.

EXAMPLE 45

A specimen of AISI 1045 steel having a diameter of 8 mm and a length of30 mm was given 150 minutes of gas soft-nitriding treatment at 570° C. Amixed powder composed of 90% of Fe-Zr having a zirconium content of 70%and 10% of KBF₄ and having a particle size under 100 mesh was placed ina stainless steel vessel and the specimen was placed in the powder. Apowder of ferroboron having a particle size under 100 mesh was placed onthe mixed powder to form an antioxidizing layer having a thickness of 3to 4 mm. The vessel was heated at 600° C. for 16 hours in an atmosphericfurnace. The vessel was taken out of the furnace and after it had beencooled by air, the specimen was removed from the powder.

The surface layer which had been formed on the specimen was examined byan X-ray microanalyzer. It was a layer composed of zirconium, nitrogenand carbon. The surface analysis of the layer showed that it containedabout 20% of zirconium. Thus, it was a layer of zirconium carbonitride.

EXAMPLE 46

A paste was prepared from a mixed powder composed of 40% of Al₂ O₃having a particle size under 200 mesh, 55% of Fe-Zr having a particlesize under 100 mesh, which had a zirconium content of 70%, and 5% of NH₄Cl having a particle size of 80 to 100 mesh by using a solvent preparedby dissolving ethyl cellulose in ethyl alcohol. A specimen of AISI W1steel having a diameter of 20 mm and a length of 10 mm was given 60hours of gas soft-nitriding treatment at 570° C. The paste was appliedto the surface of the specimen to form a layer having a thickness of 3to 5 mm thereon. Then, the specimen was placed in a stainless steelvessel and heated at 600° C. for 16 hours in an argon gas atmosphere,whereby a surface layer was formed on the specimen.

The surface layer was examined by an X-ray microanalyzer. It was a layerof zirconium carbonitride.

EXAMPLE 47

A mixed powder composed of 60% of Al₂ O₃ having a particle size under 80mesh, 38.8% of Fe-Zr having a particle size under 100 mesh, which had azirconium content of 70%, and 1.2% of NH₄ Cl having a particle sizeunder 80 mesh was placed in a fluidized bed furnace. Argon gas wasintroduced into the furnace through its bottom to fluidize the powder.The argon gas had a pressure of 1.5 kg/cm² when entering the furnace andwas caused to flow therethrough at a rate of 200 cm/min. A specimen ofAISI W1 steel having a diameter of 7 mm and a length of 50 mm, which hadbeen nitrided in a salt bath under the conditions employed in EXAMPLE 1,was placed in the furnace and heated at 600° C. for 16 hours, whereby asurface layer was formed on the specimen.

The surface layer was examined by an X-ray microanalyzer. It wascomposed of zirconium, nitrogen and carbon, as shown in FIG. 39. Thesurface analysis of the layer showed that it contained about 20% ofzirconium. The examination of the layer by X-ray diffraction gavediffraction pattern coinciding closely with that of ZrN. Thus, it was alayer of (Zr,Fe)(C,N).

EXAMPLE 48

A specimen of AISI H13 steel having a diameter of 7 mm and a length of50 mm was nitrided in a salt bath having a temperature of 570° C. forfour hours. A mixed powder composed of 58.8% of Al₂ O₃ having a particlesize under 80 mesh, 40% of Fe-Zr having a particle size under 100 mesh,which had a zirconium content of 70%, and 1.2% of NH₄ Cl having aparticle size under 80 mesh was placed in a fluidized bed furnace. Argongas was introduced into the furnace through its bottom to fluidize thepowder. The argon gas had a pressure of 1.5 kg/cm² when entering thefurnace and was caused to flow therethrough at a rate of 200 cm/min. Thespecimen was placed in the furnace and heated at 600° C. for 16 hours,whereby a surface layer was formed thereon.

The surface layer was examined by an X-ray microanalyzer. It wascomposed of zirconium, nitrogen and carbon. The surface analysis of thelayer showed that it contained about 20% of zirconium. The examinationof the layer by X-ray diffraction gave a diffraction pattern coincidingclosely with that of ZrN. Thus, it was a layer of (Zr,Fe)(C,N).

EXAMPLE 49

A specimen of AISI M2 steel having a diameter of 6.5 mm and a length of40 mm was nitrided in a salt bath under the conditions employed inEXAMPLE 38. A mixture composed of 52 mol % of CaCl₂ and 48 mol % of NaClwas placed in a vessel made of heat-resistant steel and was heated to550° C. in an atmospheric electric furnace to prepare a molten saltbath. A powder of Fe-Zr having a zirconium content of 80%, which had aparticle size under 200 mesh, was added to the bath. The amount of thepowder added was 25% of the weight of the bath. The specimen wasimmersed in the bath and heated at 550° C. for eight hours, whereby asurface layer was formed thereon. Then, it was taken out and quenched inoil.

The surface layer was examined by X-ray diffraction. It gave adiffraction pattern coinciding with that of ZrN. Thus, it was a layer ofzirconium carbonitride. This specimen will hereinafter be referred to asSpecimen No. C4.

A dry friction test was conducted on Specimen No. C4 by means of a Falexlubricant testing machine using a testing member formed from gascarburized SCM415 (JIS) steel. The test was conducted at a load of 400g, a rotating speed of 300 rpm and a friction rate of 0.1 m/sec.

For the sake of comparison, the same test was conducted on a specimen ofAISI M2 steel which had not been nitrided or heat treated (Sepcimen No.S11) and a specimen of AISI M2 steel which had been nitrided, but hadnot been heat treated (Specimen No. S12).

The results of the tests are shown in a table below. Both of SpecimensNos. S11 and S12 showed clear marks of seizing, but Specimen No. C4 wasonly negligibly damaged. Therefore, it is evident that this inventioncan form a surface layer which is superior in wear and seizingresistance to the surface of any material such as Specimen No. S11 orS12.

                  TABLE                                                           ______________________________________                                        Specimen No.                                                                              C4            S11     S12                                         Wear (mg/cm.sup.2)                                                                        5             12      9                                                     Invention   Comparative                                             ______________________________________                                    

What is claimed is:
 1. A method of treating the surface of an iron alloymaterial which comprises:nitriding said iron alloy material to form onsaid surface thereof a layer of a nitride composed of iron and nitrogenor a carbonitride composed of iron, carbon and nitrogen; and heatingsaid iron alloy material at a temperature up to 580° C. with a materialcontaining at least one surface layer-forming element selected from thegroup consisting of chromium, titanium and zirconium, and a treatingagent, so that said surface layer-forming element may be diffused intosaid surface of said iron alloy material to form thereon a surface layercomposed of a nitride or carbonitride of said element, said treatingagent being at least one compound selected from the group consisting ofchlorides, fluorides, borofluorides, oxides, bromides, iodides,carbonates, nitrates and borates of alkali and alkaline earth metals,ammonium halides and metal halides.
 2. A method as set forth in claim 1,wherein said nitriding is carried out by a method selected from gasnitriding, gas soft-nitriding, salt bath soft-nitriding and glowdischarge nitriding.
 3. A method as set forth in claim 1, wherein saidmaterial containing at least one surface layer-forming element is atleast one material selected from the group consisting of said element inpure metallic form, an alloy containing said element and a compoundcontaining said element.
 4. A method as set forth in claim 1, whereinsaid heating is carried out by immersing said iron alloy material andsaid material containing said element in a molten bath composed of saidtreating agent.
 5. A method as set forth in claim 1, wherein saidheating is carried out by employing said iron alloy material as acathode and conducting electrolysis in a molten bath composed of saidtreating agent and holding said material containing said element.
 6. Amethod as set forth in claim 1, wherein said heating is carried out byplacing said iron alloy material in a mixed powder composed of saidtreating agent and said material containing said element.
 7. A method asset forth in claim 1, wherein said heating is carried out after a pasteprepared from a mixed powder composed of said treating agent and saidmaterial containing said element has been applied to said iron alloymaterial.
 8. A method as set forth in claim 1, wherein when said ironalloy material is heated, it is kept away from contact with a mixedpowder composed of said treating agent and said material containing saidelement.
 9. A method as set forth in claim 1, wherein said iron alloymaterial is heated in a fluidized mixed powder composed of said treatingagent and said material containing said element.
 10. A method as setforth in claim 1, wherein said heating is carried out at a temperatureof at least 450° C.
 11. A method of treating the surface of an ironalloy material which comprises:nitriding said iron alloy material toform on said surface thereof a layer of a nitride composed of iron andnitrogen or a carbonitride composed of iron, carbon and nitrogen, andheating said iron alloy material at a temperature up to 580° C. with amaterial containing (a) at least one surface layer-forming element fromthe group consisting of Group Va elements of the periodic table and (b)a treating agent, so that said surface layer-forming element may bediffused into said surface of said iron alloy material to form thereon asurface layer composed of a nitride or carbonitride of said element;said treating agent being at least one compound selected from the groupconsisting of chlorides, fluorides, borofluorides, oxides, bromides,iodides, carbonates, nitrates and borates of alkali and alkaline earthmetals, ammonium halides and metal halides.
 12. A method as set forth inclaim 11, wherein said nitriding is carried out by a method selectedfrom gas nitriding, gas soft-nitriding, salt bath soft-nitriding andglow discharge nitriding.
 13. A method as set forth in claim 11, whereinsaid material containing at least one surface layer-forming element isat least one material selected from the group consisting of said elementin pure metallic form, an alloy containing said element and a compoundcontaining said element.
 14. A method as set forth in claims 11, whereinsaid heating is carried out by immersing said iron alloy material andsaid material containing said element in a molten bath composed of saidtreating agent.
 15. A method as set forth in claims 11, wherein saidheating is carried out by employing said iron alloy material as acathode and conducting electrolysis in a molten bath composed of saidtreating agent and holding said material containing said element.
 16. Amethod as set forth in claim 11, wherein said heating is carried out byplacing said iron alloy material in a mixed powder composed of saidtreating agent and said material containing said element.
 17. A methodas set forth in claim 11, wherein said heating is carried out after apaste prepared from a mixed powder composed of said treating agent andsaid material containing said element has been applied to said ironalloy material.
 18. A method as set forth in claim 11, wherein, whensaid iron alloy material is heated, it is kept away from contact with amixed powder composed of said treating agent and said materialcontaining said element.
 19. A method as set forth in claim 11, whereinsaid iron alloy material is heated in fluidized mixed powder composed ofsaid treating agent and said material containing said element.
 20. Amethod as set forth in claim 11, wherein said heating is carried out ata temperature of at least 450° C.